FRIDAY, Sept. 11, 2015 (HealthDay News) -- Long work shifts for nurses may lead to job dissatisfaction and a risk of burnout, a new study finds.

Compared with shifts that were 8 hours or less, shifts that last 12 hours or more were linked with a 40 percent higher level of job discontentment and a 31 percent higher risk of planning to quit, the study found.

"Current literature tends to report that 12-hour shifts represent a way to retain nurses in hospital clinical practice because it is believed to be the preferred shift length and that nurses are more satisfied with their jobs: our results suggest the opposite," study author Chiara Dall'Ora, from the University of Southampton, in the United Kingdom, and her colleagues wrote.

"Therefore, our findings pose substantial questions for managers, most notably because job satisfaction is a consistent and robust predictor of remaining in a job," the study authors added in the report published online Sept. 10 in BMJ Open.

Nurses commonly work longer shifts in many European countries, including England, Ireland and Poland, the study authors pointed out. Long shifts are believed to promote greater efficiency among nurses as well as greater productivity. Longer shifts also enable nurses to maintain flexible schedules, with more days off from work, the researchers said.

Burnout and job satisfaction among nurses, however, remain key global concerns since they could affect safety and the quality of patient care, the investigators explained.

The study doesn't prove that long hours cause burnout and job dissatisfaction among nurses, but the researchers said their findings challenge the idea that extended shifts improve productivity and schedule flexibility among nurses.

To look at the effects of longer shifts, researchers surveyed almost 32,000 registered nurses from nearly 500 hospitals in 12 European countries. The average age of the nurses was 38, and most were women.

Half of the study participants worked 8-hour or shorter shifts. Nearly one-third of the nurses worked shifts lasting 8 to 10 hours. Four percent worked up to 12 hours, and 14 percent worked 12 to 13 hours at a time, the study said. Only 1 percent of those polled worked shifts that lasted more than 13 hours, the study found.

About 25 percent of the nurses said they weren't satisfied with their job. A similar percentage was also not happy with the flexibility of their work schedule. One-third of the nurses surveyed said they planned to leave their job, the findings showed.

Working overtime was linked to poor outcomes in all aspects of job satisfaction -- regardless of how many extra hours they worked, the researchers said.